Enhancement: UI for editing custom fields

The UI should provide a way for users to add, remove, rename and set the data type of custom fields. Basically, to edit the "0" rows in meeting_data. This can currently be done in SQL, but I'd rather not tell users to modify their DB.

Lots of use cases for this, additional contact information, extra info about formats. It might make the UI more friendly to be able to remove parts of the address that aren't relevant in the region. In Germany in the UI we have fields for "Land" and "Nation" that people enter inconsistently.

I wouldn't call this urgent or anything, more of a nice-to-have.